1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to an image forming apparatus that sequentially operates a plurality of developing units to form a color image.
2. Description of the Related Art
A common electrophotographic image forming apparatus illuminates light onto a uniformly charged photoconductive medium to form an electrostatic latent image. The electrostatic latent image is developed using a toner to form a toner image. The developed image is transferred and fused onto a sheet of paper to form a color image. Generally, colors of yellow Y, magenta M, cyan C, and black K are used in a color image forming apparatus. Accordingly, four developing units are required for attaching the toners of the four colors onto the electrostatic latent image.
Examples of methods of forming a color image include a single-pass method in which respective four exposing units and four photoconductive mediums are provided, and a multi-pass method in which one exposing unit and one photoconductive medium are provided.
In a color image forming apparatus using the single-pass method, the time required for printing a color image is the same as in printing a black and white image. Therefore, it is mainly used in a high speed color image forming apparatus. However, the price of the apparatus becomes expensive since four exposing units and four photoconductive drums are required. To avoid this problem, in a color image forming apparatus operating at a relatively low speed, the multi-pass method is used in which one photoconductive drum and one exposing unit are provided. A color toner image is formed on an intermediate transfer medium by repeating the exposing, developing, and transferring steps with respect to each color, thereby transferring and fusing the color toner image onto a sheet of paper.
In the image forming apparatus using the multi-pass method, because four developing units are sequentially operated, a device is required for sequentially transferring a rotational force of a driving motor. To this end, a conventional image forming apparatus has used four electrical clutches. However, the electrical clutches are expensive and large. Additionally, because sliding may occur during clutching, there has been a problem in that a driving force cannot be timely regulated.
Accordingly, a need exists for an image forming apparatus having improved regulation of the driving force transferred to a developing unit.